The connection is returned to the free pool when sendMessage() exits.

当 sendMessage() 退出时，该连接返回到空闲池。

The sendMessage() function creates an AsyncMessage, which is an implementation of the IMessage interface, and then sends the message using the producer’s send() method.

sendMessage()函数先创建一个AsyncMessage，其是IMessage接口的一个实现，然后使用producer的send()方法发送此消息。